The SFRM&HS' SANTA FE OPEN-TOP CARS, by Richard Hendrickson, has photos of the GA-53 cars on pages 146-148. While most of the underside stuff is hidden in the shadows, a couple of the photos do show what may be the end of the brake reservoir that is mounted perpendicular to the center sill.

Cheers.
Bill Keene
Irvine, CA